This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Faithful chromosome segregation relies on the cellular mechanism that monitors microtubule-kinetochore attachment. The correct attachment allows cells to progress to the next cell cycle, while errors cause cell cycle arrest until they become corrected. The signal to trigger this arrest is either lack of attachment between kinetochore and microtubule or lack of tension in the spindle. This signaling is known to involve spindle checkpoint proteins and other mitotic kinases and phosphatases. However, how these components function coordinately to regulate the cell cycle has not been fully understood, partly because many mitotic kinases conduct multiple functions throughout the cell cycle. To address this issue, we are conducting affinity purification and mass spectrometric analysis of mitotic kinase complexes from different cells cycle stages and in different states of microtubule-kinetochore attachment to identify any differences in protein-protein interactions and post-translational modifications. This will allow us to understand how these complexes are regulated in response to errors in microtubule-kinetochore attachment.